Leg length calculation in computer-assisted surgery

ABSTRACT

A computer-assisted surgery (CAS) system outputs a leg length discrepancy and/or an offset between conditions. An inertial sensor unit is connected to an instrument(s) to produce readings representative of its orientation. A CAS processor unit has a coordinate system module for setting a pelvic coordinate system from readings of the inertial sensor unit, a tracking module for tracking an orientation of the instrument(s) relative to the pelvic coordinate system during movements thereof, and a geometrical relation data module for recording preoperatively a medio-lateral orientation of the instrument(s) representative of a medio-lateral axis of the legs and a distance between the legs, for recording after implant rejointing the medio-lateral orientation and the distance, and for calculating a leg length discrepancy and/or an offset, based on the distances and the medio-lateral orientations. An interface outputs the leg length discrepancy and/or the offset between leg conditions.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the priority of U.S. Provisional PatentApplication No. 62/110,861, filed on Feb. 2, 2015, and incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure relates to a system and method used inComputer-Assisted Surgery (CAS) to provide leg length discrepancy andoffset measurements, for instance in hip surgery.

BACKGROUND OF THE ART

In orthopedic surgery, for instance hip replacement, leg lengthdiscrepancy is a change of leg length along the longitudinal axis of thepatient, between a preoperative length and an intra-operative orpost-operative length. Also in hip replacement, offset is themeasurement of the translational shift of the leg along a medio-lateralaxis of the patient, at the hip joint. Both these parameters arerelevant during hip surgery, including total hip replacement, acetabularcup implanting, femoral implanting (e.g., head and neck implant,resurfacing). Hence, there is a need for systems and methods fordetermining leg length discrepancy and offset that is minimally invasiveyet precise and accurate.

SUMMARY

It is aim of the present disclosure to provide novel systems and methodsfor determining leg length discrepancy and offset to assess orthopedichip surgery.

Therefore, in accordance with a first embodiment of the presentdisclosure, there is provided a computer-assisted surgery system foroutputting at least one of a leg length discrepancy and an offsetbetween a preoperative leg condition and a post-implant rejointing legcondition comprising: at least one instrument; at least one inertialsensor unit connected to the at least one instrument, the inertialsensor unit producing readings representative of its orientation; acomputer-assisted surgery processor unit operating a surgical assistanceprocedure and comprising a coordinate system module for setting a pelviccoordinate system from readings of the at least one inertial sensor unitwhen the at least one instrument is in a given orientation relative tothe pelvis, a tracking module for tracking an orientation of the atleast one instrument relative to the pelvic coordinate system duringmovements thereof using the readings from the inertial sensor unit onthe instrument, and a geometrical relation data module for recordingpreoperatively a medio-lateral orientation of the at least oneinstrument representative of a medio-lateral axis of the legs relativeto the pelvic coordinate system and a distance between the legs alongthe medio-lateral axis, for recording after implant rejointing themedio-lateral orientation and said distance, and for calculating atleast one of a leg length discrepancy and an offset, based on saiddistances and said medio-lateral orientations; an interface foroutputting at least the leg length discrepancy or the offset between thepreoperative leg condition and the post-implant rejointing legcondition.

Further in accordance with the first embodiment, the at least oneinstrument is a caliper having a body with a translational joint forexpanding/contracting, and legs configured for abutment with pelviclandmarks.

Still further in accordance with the first embodiment, the at least oneinstrument supports a light source emitting a light beam that isperpendicular relative to a direction of the translational joint.

Still further in accordance with the first embodiment, the light sourceis displaceable along the body, the light beam being a leg alignmentmarker when the caliper is abutted against the pelvic landmarks.

Still further in accordance with the first embodiment, the givenorientation has a direction of the translational joint parallel to amedio-lateral axis of the pelvis.

Still further in accordance with the first embodiment, an ankle clamphas ankle interfaces configured to remain fixed to the ankles, withlinkages interconnecting the ankle interfaces.

Still further in accordance with the first embodiment, a scale in thelinkages measures the distance.

Still further in accordance with the first embodiment, the linkagesinclude at least a translational joint in a direction generally alignedwith a medio-lateral axis between the legs.

Still further in accordance with the first embodiment, indicators areprovided for receiving ends of the caliper for recording themedio-lateral orientation with the caliper abutted against the ankleclamp.

Still further in accordance with the first embodiment, the at least oneinstrument is an acetabular-implant impactor, and wherein the impactorsupports a light source emitting a light beam having a known orientationrelative to a longitudinal axis of the impactor.

Still further in accordance with the first embodiment, the givenorientation has the light beam illuminating the medio-lateral axis ofthe pelvis, with a shaft of the impactor lying in a plane of the lightbeam.

Still further in accordance with the first embodiment, an ankle clamphas ankle interfaces configured to remain fixed to the ankles, withlinkages interconnecting the ankle interfaces, the ankle clamp furthercomprising indicators for being illuminated by the light beam forrecording the medio-lateral orientation.

Still further in accordance with the first embodiment, a scale is in thelinkages to measure the distance.

In accordance with a second embodiment of the present disclosure, thereis provided a computer-assisted surgery system for outputting at leastone of a leg length discrepancy and an offset between a preoperative legcondition and a post-implant rejointing leg condition comprising: atleast one instrument; at least one inertial sensor unit connected to theat least one instrument, the inertial sensor unit producing readingsrepresentative of its orientation; a computer-assisted surgery processorunit operating a surgical assistance procedure and comprising acoordinate system module for setting a pelvic coordinate system fromreadings of the at least one inertial sensor unit when the at least oneinstrument is in a given orientation relative to the pelvis, a trackingmodule for tracking an orientation of the at least one instrumentrelative to the pelvic coordinate system during movements thereof usingthe readings from the inertial sensor unit on the instrument, and ageometrical relation data module for recording preoperatively a landmarkorientation relative to the pelvic coordinate system and a distance whenthe at least one instrument has a first end abutted to a pelvic landmarkand a second end abutted to a leg landmark, for recording after implantrejointing the landmark orientation and said distance, and forcalculating at least one of a leg length discrepancy and an offset,based on said distances and said landmark orientations; an interface foroutputting at least the leg length discrepancy or the offset between thepreoperative leg condition and the post-implant rejointing legcondition.

Further in accordance with the second embodiment, the at least oneinstrument is a caliper having a body with a translational joint forexpanding/contracting, and legs configured for contacting the pelviclandmark and the leg landmark.

Still further in accordance with the second embodiment, the calipersupports a light source emitting a light beam that is perpendicularrelative to a direction of the translational joint.

Still further in accordance with the second embodiment, the givenorientation has the light beam illuminating the medio-lateral axis ofthe pelvis.

Still further in accordance with the second embodiment, a scale is onthe translational joint to obtain said distances.

Still further in accordance with the second embodiment, the at least oneinstrument includes a mechanical gauge having body with a translationaljoint for expanding/contracting, and bores configured for beingconnected to pins constituting the pelvic landmark and the leg landmark.

Still further in accordance with the second embodiment, a scale is onthe translational joint to obtain said distances.

Still further in accordance with the second embodiment, the at least oneinstrument includes an acetabular-implant impactor supporting theinertial sensor unit, and wherein the impactor supports a light sourceemitting a light beam having a known orientation relative to alongitudinal axis of the impactor.

Still further in accordance with the second embodiment, the givenorientation has the light beam illuminating the medio-lateral axis ofthe pelvis, with a shaft of the impactor lying in a plane of the lightbeam.

Still further in accordance with the second embodiment, the landmarkorientation has the light beam illuminating a longitudinal axis of themechanical gauge, with a shaft of the impactor lying in a plane of thelight beam.

In accordance with the third embodiment of the present disclosure, thereis provided a method for repeating a leg alignment between apreoperative leg condition and a post-implant rejointing leg condition,comprising: pre-operatively, with the patient in supine decubitus,orienting a light source using landmarks on the pelvis to produce alight beam aligned with a transverse plane of the pelvis, positioning atleast one of the legs of the patient in alignment with the light beam,and setting landmarks on the legs of the patient, distally from thepelvis; post post-implant rejointing, with the patient in supinedecubitus, repeating the orienting and the positioning, and noting amovement of the landmarks.

Still further in accordance with the third embodiment, setting landmarkson the legs of the patient comprises projecting a light beam from alandmark on a first of the legs onto a scale on a landmark on a secondof the legs.

Still further in accordance with the first embodiment, noting a movementof the landmarks comprises at least noting a displacement of the lightbeam on the scale.

Still further in accordance with the first embodiment, wherein noting amovement of the landmarks comprises at least noting a variation ofdistance between the landmarks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a caliper instrument on a pelvis duringa leg positioning technique;

FIG. 2 is a perspective view of the caliper instrument on a mechanicalankle clamp;

FIG. 3 is a perspective view of an impactor using in leg length andoffset measurement relative to a pelvis;

FIG. 4 is a perspective view of an impactor using in leg length andoffset measurement relative to the mechanical ankle clamp;

FIG. 5 is a perspective view of a pinned mechanical gauge;

FIG. 6 is an enlarged perspective view of the pinned mechanical gauge;

FIG. 7 is a perspective view of a pinned mechanical gauge and impactor;

FIG. 8 is a perspective view of the pinned mechanical gauge andimpactor;

FIG. 9 is a perspective view of the pinned mechanical gauge and caliperinstrument;

FIG. 10 is an enlarged view of the scale on the caliper instrument;

FIG. 11 is a perspective view of the mechanical ankle clamp with lightsource;

FIG. 12 is an enlarged view of a scale on the mechanical ankle clamp;and

FIG. 13 is a block diagram showing a computer-assisted surgery systemoperating with instruments to calculate leg length discrepancy andoffset, in accordance with the present disclosure.

DETAILED DESCRIPTION

In the proposed disclosure, the leg length discrepancy and offsetmeasurements are resolved using basic trigonometry. Leg lengthdiscrepancy and/or offset are measured to quantify the post-operativegait of the patient, to diagnose a patient condition, to assist in aphysiotherapy treatment, or even to perform corrective actionsintra-operatively, among numerous other possibilities. The measurementsmay be performed on a patient during hip replacement surgery, or can beperformed on a bone model or cadaver. In general, the distancemeasurements are obtained based on the readings from mechanicalinstruments. The use of inertial sensors may assist in giving precisionand accuracy to the afore-mentioned measurements. For example, as shownin FIG. 1, a caliper instrument 10 may be used. The caliper instrument10 is described in US Patent Application Publication No. 2014/0031829,incorporated herein by reference, and uses inertial sensor technology.

As shown in FIG. 1, the caliper instrument 10 may be used as part of abone digitizer in a bone digitizing system, to create a frame ofreference for subsequent navigation of tools relative to bones insurgery, for instance based on the determination of the medio-lateralaxis of the pelvis. The instrument 10 is referred to as a caliper, as itfeatures a pair of legs 12 movable relative to one another, e.g., in atelescopic manner. The expression “caliper” is used nonrestrictively.Any other appropriate expression may be used to describe the instrument10, such as medio-lateral digitizer.

In the illustrated embodiment, the legs 12 of FIG. 1 each comprise atranslational joint 13 so as to be expandable or contractible along theY axis. For instance, the translational joints 13 may be any of slidingjoint, telescopic joint, prismatic joint, indexing joint, etc. As analternative, a single one of the legs may have a joint. It is alsoconsidered to use rotational joints as an alternative to translationaljoints 13, with an axis of the rotational joint being normal to a planeof the caliper instrument 10. A locking mechanism is typically provided,to lock the translational joints 13 and, therefore, set the legs 12 in aselected length. The free end of each leg 12 has an abutment end 14, forwhich any appropriate shape is considered, such as flat contactsurfaces, discs, various concavities or convexities, pointy ends, etc.,as a function of the type of bone or bodily part the caliper instrument10 will be contacting. The flat ends 14 of FIG. 1 are well suited to beused with a pelvis, with the ends 14 contacting the anterior superioriliac spines (ASIS) on opposite sides of the pelvis, in pelvic surgery,with the patient in supine decubitus. Alternatively, the caliperinstrument 10 could be used for the posterior superior iliac spine aswell, among other possibilities.

Still referring to FIG. 1, the legs 12 are interconnected by anelongated body 20 of the caliper instrument 10. The elongated body 20features a translational joint 21 such that the elongated body 20 isexpandable or contractible along the X axis. The translational joint 21may be any appropriate joint, such as translational joints, telescopicjoint, prismatic joints and/or indexing joints. It is also considered touse rotational joints as an alternative to the translational joint 21.

A locking mechanism may be provided, thereby allowing the user to setthe length of the elongated body 20. An inertial sensor support orreceptacle 23 is defined on the elongated body 20. The inertial sensorsupport 23 is, for instance, made with a specific geometry in order toprecisely and accurately accommodate an inertial sensor unit in apredetermined complementary connection, simplifying an initializationbetween an inertial sensor unit 26 (FIG. 2) and caliper instrument 10.For instance, the inertial sensor unit has a preset orientation that isaligned with a dimension of the caliper instrument 10. In other words,the mechanical constraints in the attachment of inertial sensor unit inthe support 23 are such that the three axes of the inertial sensor unitare aligned with the X, Y and Z axis of the caliper instrument 10.Therefore, the caliper instrument illustrated in FIG. 1 may expand andcontract along both the X axis and the Y axis. A light source 24 is alsoprovided on the caliper instrument 10. The light source 24 is of thetype producing a planar beam, such that a projection of the planar beamon a surface produced a line. The light source 24 may be on a carriage25 so as to be displaceable in translation along the elongated body 20.Alternatively, it is considered to configure the carriage 25 to besnap-fitted to the elongated body 20, so as to allow its installation atany position along the elongated body 20.

The inertial sensor unit 26 used with the caliper instrument 10 may haveany appropriate type of inertial sensor, to provide 3-axis orientationtracking. For instance, the inertial sensor unit may have sets ofaccelerometers and/or gyroscopes, etc. The inertial sensor unit may beknown as a sourceless sensor unit, as a micro-electromechanical sensorunit, etc. As mentioned above, the inertial sensor unit is matinglyreceived in the inertial sensor support 23 in a predeterminedcomplementary connection, such that the initializing of the inertialsensor unit will have the inertial sensor unit specifically orientedrelative to the X-Y-Z coordinate system illustrated in FIG. 1 (with theZ axis being the cross-product of the X and Y axes).

The inertial sensor unit 26 uses inertial sensor readings to continuallycalculate the orientation and velocity of a body without the need for anexternal reference, i.e., no signal transmission from outside of thesensor assembly is necessary, the inertial sensor unit 26 isself-contained. This process is commonly known as dead-reckoning andforms part of the common general knowledge. An initial orientation andvelocity must be provided to the inertial sensor unit 26, i.e., theX-Y-Z coordinate system of FIG. 1, after which the orientation istracked by integrating the angular rates of gyroscope readings at eachtime step. With an accurate estimate of the orientation of the inertialsensor unit 26 with respect to the Earth frame of reference,gravitational effects can be removed and inertial forces acting on theaccelerometers can be integrated to track changes in velocity andposition. Since the inertial sensor unit 26 has no need for an externalreference, it may be immune to environmental factors such as magneticfields and operate under a wide range of conditions.

Referring to FIG. 2, a mechanical clamp 30 is illustrated. Themechanical clamp 30 has ankle hoops 31 or like ankle attachments orinterfaces, separated by a lockable translation joint 32. Hence, adistance between the ankle hoops 31 may be adjusted. The distancebetween the two ankle hoops 31 can be read from a scale on the joint 32.The ankle hoops 31 are illustrated as being inverted U-shapedstructures. According to an embodiment, the hoops 31 each abut againstthe pair of ankle malleoli, such that the interconnection between thehoop 31 and respective ankle is stable and reproducible. For thispurpose, the hoop 31 may have cavities 31A to accommodate the malleoli.Other configurations are considered, including different shapes for thehoops 31, with straps, other joint sets, etc.

The mechanical clamp 30 may have visual indicators 33 to receive thereinthe ends 14 of the caliper instrument 10 in the manner shown in FIG. 2,to use the scale of the caliper instrument 10, and also ensure preciseand reproducible alignment between caliper instrument 10 and mechanicalclamp 30, such that the interconnection between the caliper instrument10 and the mechanical clamp 30 is reproducible from a pre-operative to apost-operative interaction. The visual indicators 33 may identify thecenter of two malleoli on both ankles, when the mechanical clamp 30 isused. Moreover, the ankle hoops 31 may translate longitudinally withrespect to one another (i.e., along the leg), by way of lockabletranslational joint 34. Other types of joints (i.e., linkages) may alsobe used to allow relative movement between the ankle hoops 31 and thelockable translation joint 32. For example, the lockable translationjoint 32 may have hinges at its ends, by which it would be connected tothe ankle hoops 31. Accordingly, the ankle interfaces 31 may remain in afixed relation with the ankles, while the various joints describedherein allow relative movement between the ankles. The visual indicators33 are positioned such that any relative movement between apre-operative condition and a post implant rejointing condition can bequantified as described below.

Referring to FIGS. 5, 6, 7 and 8, a mechanical gauge in accordance withthe present disclosure is shown at 40, and is another of the instrumentsthat may be used to implement the method of the present disclosure. Themechanical gauge 40 is of the type using a pair of pins 41, though pinholes 42 located at opposed ends of the mechanical gauge 40. A scale 43is provided on a lockable translational joint 44 of the gauge 40.Accordingly, the mechanical gauge 40 can be used to measure distances.In an embodiment, the mechanical gauge 40 is biased to a zero reading onthe scale 43.

Referring to FIGS. 3, 4, 7 and 8, an impactor is shown at 50. Theimpactor 50 is of the type used in impacting an acetabular cup implantin the acetabulum, for instance as described in PCT InternationalPublication No. WO 2014/197988, incorporated herein by reference. Theimpactor 50 may be used as one of the instruments to measure the leglength discrepancy and the offset, for the simple reason that mayalready be used for the implant procedure. The impactor 50 has the lightsource 51 allowing its alignment, and an inertial sensor unit 52 similarto the unit 26, containing a gyroscope for dead-reckoning.

Referring to FIG. 13, a system for navigating the instruments describedabove in computer-assisted hip surgery is generally shown at 100, and isof the type used to implement the method detailed below. In anembodiment, the system 100 is used for assisting the user in performinghip surgery, but also has the modules to perform the leg lengthdiscrepancy and offset calculations described herein. The system 100comprises a computer-assisted surgery (CAS) processing unit 102. The CASprocessing unit 2 may be integrated into one or more inertial sensorunits 26 and 52, also known as pods that are mounted to the variousinstruments of the system 100, or as a module of a computer or portabledevice, among other possibilities.

The inertial sensor units 26 and 52 incorporate the processing unit 102and may thus be equipped with a user interface(s) 103 to provide thenavigation data, whether it be in the form of LED displays, screens,numerical displays, etc. Alternatively, the inertial sensor unit 26 and52 may be connected to a stand-alone processing device B that wouldinclude a screen or like monitor, to provide additional display capacityand surface. By way of example, the processing device B is a wirelessportable device such as a tablet in a wired or wireless communicationwith the inertial sensor unit 26/52.

The inertial sensor unit 26/52 may be known as micro-electro-mechanicalsensors (MEMS) and may include one or more of inertial sensors, such asaccelerometers, gyroscopes, magnetometers, among other possible inertialsensors. The inertial sensors are sourceless sensors automaticallyproviding data influenced by natural phenomena, such as gravity. Theinertial sensor unit A also have a body, typically defined by a casing,giving the inertial sensor unit A, by which the inertial sensor unit Amay be secured to the instruments.

The processing unit 102 comprises different modules to perform thenavigation. A surgical flow module 102A may be used in conjunction withthe user interface 103 or a processing device B to guide the operatorthrough the steps leading to the navigation. This may entail providing astep-by-step guidance to the operator, and prompting the operator toperform actions, for instance pressing on a “record” interface that ispart of the interface 103 or entering data as measured from the scalesof the caliper instrument 10 or mechanical gauge 40, for the system 100to record instant orientations and position data. While this occursthroughout the surgical procedure, the prompting and interactionsbetween the system 100 and the user will not be described in a remainderof the description, as they will implicitly occur. It is contemplated tohave the surgical flow module 102A present in the processing device B,with concurrent action between the inertial sensor unit A and theprocessing device B to guide the operator during the measuringprocedures detailed below, and with a communication with the operator torecord the progress of the procedure.

A tracking module 102B may also be part of the processing unit 102. Thetracking module 102B receives readings from the inertial sensors 26/52,and converts these readings to useful information, i.e., the navigationdata. As described above, the navigation data may be orientation datarelating an instrument to the pelvis. The tracking module 102B mayperform dead-reckoning to track the inertial sensors 26/52, as describedbelow.

The coordinate system module 102C creates the coordinate system. Thecoordinate system is the virtual frame by which the orientation of theinstruments and tools is related to the orientation of the bone. Forexample, the coordinate system module 102C sets a pelvic coordinatesystem from readings of the inertial sensor 26/52 when instruments arein a given orientation relative to the pelvis.

In order to output the record orientations at discrete desiredorientations and calculate offset and leg length discrepancy, via theuser interface 103 or processing device B, the processing unit 102 maybe preprogrammed with geometrical relation data module 102D. Thegeometrical relation data module 102D is therefore used to recordorientations of the various instruments supporting the inertial sensors26/52, and uses these orientations along with distances to calculate theleg length discrepancy and/or the offset.

The inertial sensor units 26/52 are designed such that they areconnected in single possible orientation to the instruments and tools,such that the orientation of the inertial sensor units 26/52 is knownrelative to the instruments and tools to which it is connected whenturned on. By way of the connector 5, the inertial sensor units A may beportable and detachable units, used with one device/instrument, and thentransferred to another device/instrument, preserving in the processorientation data of the global coordinate system, using dead-reckoning.

The geometrical relation data module 102D is programmed for specific usewith the devices and instruments described herein. Accordingly, when aninertial sensor unit is mounted to one of the devices and instruments,the relation between the device/instrument and a coordinate system ofthe inertial sensor unit is known (in contrast to a global coordinatesystem) by the geometrical relation data module 102D. For example, therelation may be between an axis or a 3D coordinate system of thedevice/instrument and the coordinate system of the inertial sensor unitA.

The navigation of instruments is intended to mean tracking at least someof the degrees of freedom of orientation in real-time or quasi-realtime, such that the operator is provided with navigation data calculatedby computer assistance. The inertial sensors A used in the followingmethod may be interrelated in the global coordinate system (hereinafter,coordinate system), provided appropriate steps are taken to record orcalibrate the orientation of the inertial sensors A in the coordinatesystem. The coordinate system serves as a reference to quantify therelative orientation of the different items of the surgery, i.e., theinstruments and devices relative to the pelvis.

The present application contemplates different techniques to provide theleg length and offset measurements. In general, the techniques eachcomprise two procedures, i.e., leg positioning, and taking the leglength and/or offset measurements. The following paragraphs set outdifferent techniques to measure leg length discrepancy and offset,between a pre-operative condition, and a post-operative condition, usingsome of the instruments described above. For clarity, the expressionpost-operative is used herein as representative of a part of theprocedure after positioning of the implant on the bone, when the leg canbe rejointed, i.e. post-implant rejointing. However, post-operativeincludes intra-operative, in that the measurements may be taken beforethe end of the procedure, to allow corrective measures to be taken, forexample. Hence, throughout the text, the use of the expression“post-operative” includes intra-operative interventions. The techniquesthat do not use the mechanical gauge 40 are non-invasive, in that theymay be used over the skin, or in that they do not require patient tissuealterations other than the ones required for surgery.

Procedure of Leg Positioning

The purpose of this procedure is to position or reposition the leg alongthe longitudinal axis of the patient (a.k.a., cranial-caudal axis), in areproducible manner. If the leg is laid flat on the table, this legpositioning may enable alignment of the leg with the frontal plate ofthe patient. In order to measure offset and leg length discrepancyprecisely and accurately, the leg positioning must be replicated betweenmeasurements. The impact on the measurements of the leg lengthdiscrepancy introduced by misalignment of the leg is minimized by theuse of this procedure. The procedure is performed as follows:

-   -   1. The patient is placed in supine decubitus.    -   2. Referring to FIG. 1, the caliper instrument 10 is placed on        two pelvic landmarks, after being telescopically arranged to        have a suitable length. For example, the caliper instrument 10        is placed on the two anterior-superior iliac spines, in the        manner shown in FIG. 1. An assumption is made that the caliper        instrument 10 is aligned with the medio-lateral axis of the        patient. A light beam is shone from the light source 24 that is        attached to the caliper instrument 10. The light source 24 is        connected to the frame of the caliper instrument 10 such that        the light beam is projected distally and perpendicular to the        frame of the caliper instrument 10, and therefore parallel to        the longitudinal axis of the patient, a.k.a., the cranial-caudal        axis, in direction Z of FIG. 1. The user is required to align        the first leg with the projected light beam, by manually        displacing it.    -   3. Different approaches are considered for the alignment. For        instance, as the light beam produces a line, the user may align        the light beam line with leg landmarks. For example, a center of        the knee cap and a center of the ankle to be shone by the light        beam line. Temporary pen or ink markings may be made on the knee        and/or ankle to indicate the landmarks used for alignment.    -   4. The light source 24 is then slid along the caliper instrument        10, using carriage 25. The light beam is therefore translated        laterally. As a result, the second leg can be aligned in the        same way, by manually displacing it, as guided by the selected        landmarks. Since the light beam is perpendicular to the caliper        instrument 10—and hence also perpendicular to the medio-lateral        axis of the patient—, the light beam indicates the projection of        the sagittal plane on the patient. As an alternative to assuming        this, the table plane can be assessed by a pod to determine if        the table plane is leveled. Once aligned using the light source        24 in the manner described above, the assumption is made that        the legs are physically aligned with respect to the longitudinal        axis. Moreover, as the patient is in supine decubitus, it can be        assumed that the legs are within the frontal plane. As a result,        the leg is along the longitudinal axis (i.e. the intersection of        both sagittal and frontal planes). Based on these assumptions,        the leg length discrepancy can be measured along the        longitudinal axis. The offset can be measured along the        medio-lateral axis. This is achieved by comparing data obtained        from the instruments described above, between pre-operative        measurements, and intra-operatively and/or post-operative        measurements.

Procedure: Leg Length Discrepancy and/or Offset Measurements

Numerous techniques are possible for this procedure, as described belowwith reference to the figures.

Technique 1: the instruments required are the caliper instrument 10, oralternatively the impactor 50, with light source 24 and dead-reckoningof the inertial sensor unit 26 or 52, to measure leg length discrepancy.

-   -   1. Referring to FIG. 2, with the patient's legs positioned using        the leg positioning procedure mentioned above, the mechanical        clamp 30 is rigidly attached to the ankles of the patient;    -   2. The first medio-lateral axis, i.e., that of the pelvis, is        acquired by using the caliper instrument 10 in the manner        described above, or the impactor 50. The impactor 50 may be        navigated to determine the medio-lateral axis, for instance as        described in PCT International Publication No. WO 2014/197988,        incorporated herein by reference. For example, the light beam of        the light source is in a known relation relative to a shaft of        the impactor 50;    -   3. After acquiring the pelvic medio-lateral axis, the caliper        instrument 10 is moved to the ankles to acquire a second        medio-lateral axis, near the feet. For example, the second        medio-lateral axis, i.e., the leg medio-lateral axis, may be        defined by the line connecting the two centers of both ankles        (as in FIG. 2), thus making use of the visual indicators of the        mechanical clamp 30 to physically provide these landmarks. In        the arrangement of FIG. 2, the caliper instrument 10 is in a        position to record the medio-lateral axis at the ankle;    -   4. In the acquisition of the medio-lateral axes, the inertial        sensor unit 26 attached to the caliper instrument 10 (or        impactor in alternative embodiment) contains a gyroscope. The        gyroscope will provide data that is then used by a CAS        processing unit to perform dead-reckoning and hence acquire the        relative orientation between the two medio-lateral axes,        α_(preop), i.e., at the hip (FIG. 1) and at the ankles (FIG. 2);    -   5. During or upon finishing the hip surgery, with the operated        leg rejointed, the angle α_(postop) between two medio-lateral        axes is obtained by repeating steps 1-4. The same leg        positioning technique is used prior to taking the measurements        to ensure the legs are positioned in the same way as        preoperatively, i.e., parallel to the sagittal plane;    -   6. Based on the known distance between the two ankles (D) as        obtained from the scale on the joint 32 (e.g., scale 21) and the        angular difference in α, the leg length discrepancy can be        resolved as: D·tan(α_(postop))−D·tan(α_(preop)). A positive        value would mean a longer leg post-operatively, whereas a        negative value would mean a shorter leg post-operatively. FIGS.        3 and 4 illustrate technique 1 using the impactor alternative.        It should be noted that D may vary between a pre-operative        measurement and a post-operative measurement, whereby the first        D in the solution is D measured post-operatively and the second        D in the solution is D measured pre-operatively. It is also        contemplated to fix the D, whereby step 5 would not require        repositioning the leg as in step 1.

Technique 2: caliper instrument 10 is used for this technique, tomeasure the offset.

-   -   1. The patient's legs are positioned using the leg positioning        technique described above, sliding the light source 24 on the        caliper instrument 10 to align the projected light beam on both        legs, as in FIG. 1;    -   2. The readings from the inertial sensor unit 26 on the caliper        instrument 10 are recorded (α_(preop));    -   3. Upon finishing the hip surgery, steps 1-2 are repeated to        acquire the readings (α_(postop)) from the inertial sensor unit        on the caliper instrument 10;    -   4. The offset can be resolved as: Ω_(postop)−Ω_(preop), positive        value indicates an increase in the offset and negative value        indicates a decrease in the offset.

Technique 3: this technique uses the mechanical measuring gauge 40 anddead-reckoning.

-   -   1. The patient's legs are positioned using the leg positioning        technique.    -   2. Prior to cutting the femoral neck and preparation of the        acetabulum, as shown in FIGS. 5 and 6, a first pin 41 is fixed        on the ASIS and another pin 41 is fixed on the greater        trochanter area; both pins 41 are on the operated side. The pins        41 respectively constitute a pelvis landmark and a leg landmark;    -   3. The mechanical gauge 40 is fixed to the two pins 41, and the        distance M between the two pins 41 is known from the scale 42 of        the gauge 40;    -   4. The impactor 50 as shown in FIG. 7 is used and firstly        aligned with the medio-lateral axis (using the light source 51        thereof to project a light beam on the two ASIS); then, the        impactor 50 is aligned using the light source 51 with the long        axis of the mechanical gauge 40, as in FIG. 8, showing a        landmark orientation. The inertial sensor unit 52 containing a        gyroscope to perform dead-reckoning to acquire the angle        (β_(preop), note β_(preop)<π/2) between the medio-lateral axis        and the long axis of the mechanical gauge 40;    -   5. The gauge 40 is removed, while the pins 41 are kept on the        femur and pelvis, at which point the user may proceed with the        femoral procedure;    -   6. Upon finishing placing the femoral implant and/or the        acetabular component and rejointing the leg (intra operatively        or post operatively), the distance of the gauge 40 is adjusted,        and the gauge 40 is reattached to the two pins 41. The angle        (β_(postop), note β_(postop)<π/2) are acquired between the        medio-lateral axis and the long axis of the gauge 40 by        repeating step 4-5; The same leg positioning procedure is used        beforehand to make sure the legs are positioned in the same way        as preoperatively;    -   7. The leg length discrepancy can be resolved as:        M·sin(β_(postop))−M·sin(β_(preop)); The offset can be resolved        as: M·cos(β_(postop))−M·cos(β_(preop)). It should be noted that        M may vary between a pre-operative measurement and a        post-operative measurement, whereby the first M in the solution        is M measured post-operatively and the second M in the solution        is M measured pre-operatively.

Technique 4: this technique involves the caliper instrument 10 for adirect measurement of leg length discrepancy (proximal)

-   -   1. The patient's legs are positioned using the leg positioning        procedure described above;    -   2. The ends 14 of the caliper instrument 10 are placed on the        ASIS of the operated side and on a marked reference on the skin        on the femur (e.g., a landmark on the skin), as shown in FIGS. 9        and 10, respectively the pelvic landmark and the leg landmark.        The light source 24 is displaced to project its beam on the        opposite ASIS, when selecting the marked reference;    -   3. The initial distance measurement is recorded on the caliper        instrument 10 (N_(preop));    -   4. Upon finishing the surgery, the distance measurement is        obtained using the caliper instrument 10 (N_(postop)), after        repeating the leg positioning procedure, and by repeating steps        2 and 3;    -   5. The leg length discrepancy can be resolved as:        N_(postop)−N_(preop).

Technique 5: Direct measurement of leg length discrepancy (distal),using the caliper instrument 10, the mechanical clamp 30 and using oneof the light sources 24 or 51.

-   -   1. The patient's legs are positioned using the leg positioning        procedure described above;    -   2. The mechanical clamp 30 is placed on both ankles, in the        manner shown in FIG. 2;    -   3. The light source 51 is connected to the mechanical clamp 30,        and is projected on a scale 70 on the operated leg to record the        initial leg length (X_(preop)), as shown in FIG. 11. Surgery may        be initiated, however, the translational joint 34 is unlocked to        allow the translation of the ankle hoops 31 relative to one        another;    -   4. Upon finishing the surgery, the distance measurement is        obtained using the mechanical clamp 30 (X_(postop)) by repeating        the steps 1-3; and    -   5. The leg length discrepancy is resolved as:        X_(postop)−X_(preop).

The invention claimed is:
 1. A computer-assisted surgery system foroutputting one of a leg length discrepancy and/or an offset between apre-implanting leg condition and a post-implant rejointing leg conditioncomprising: at least one instrument; at least one inertial sensor unitconnected to the at least one instrument, the inertial sensor unitproducing readings representative of its orientation; acomputer-assisted surgery processor unit operating a surgical assistanceprocedure and comprising a coordinate system module for setting a pelviccoordinate system from readings of the at least one inertial sensor unitwhen the at least one instrument is in a given orientation relative tothe pelvis, a tracking module for tracking an orientation of the atleast one instrument relative to the pelvic coordinate system duringmovements thereof using the readings from the inertial sensor unit onthe instrument, and a geometrical relation data module for recordingbefore implanting an implant a medio-lateral orientation of the at leastone instrument representative of a medio-lateral axis of the legsrelative to the pelvic coordinate system and a distance between the legsalong the medio-lateral axis, for recording after implant rejointing themedio-lateral orientation and said distance, and for calculating a leglength discrepancy and/or an offset, based on said distances and saidmedio-lateral orientations; an interface for outputting the leg lengthdiscrepancy and/or the offset between the pre-implanting leg conditionand the post-implant rejointing leg condition.
 2. The computer-assistedsurgery system according to claim 1, wherein the at least one instrumentis a caliper having a body with a translational joint forexpanding/contracting, and legs configured for abutment with pelviclandmarks.
 3. The computer-assisted surgery system according to claim 2,wherein the at least one instrument includes a light source emitting alight beam that is perpendicular relative to a direction of thetranslational joint.
 4. The computer-assisted surgery system accordingto claim 3, wherein the light source is displaceable along the body, thelight beam being a leg alignment marker when the caliper is abuttedagainst the pelvic landmarks.
 5. The computer-assisted surgery systemaccording to claim 2, wherein the given orientation has a direction ofthe translational joint parallel to a medio-lateral axis of the pelvis.6. The computer-assisted surgery system according to claim 2, furthercomprising a mechanical clamp having ankle interfaces configured toremain fixed to the ankles, with linkages interconnecting the ankleinterfaces.
 7. The computer-assisted surgery system according to claim6, further comprising a scale in the linkages to measure the distance.8. The computer-assisted surgery system according to claim 6, whereinthe linkages include at least a translational joint in a directiongenerally aligned with a medio-lateral axis between the legs.
 9. Thecomputer-assisted surgery system according to claim 6, furthercomprising indicators for receiving ends of the caliper for recordingthe medio-lateral orientation with the caliper abutted against themechanical clamp.
 10. The computer-assisted surgery system according toclaim 1, wherein the at least one instrument is an acetabular-implantimpactor, and wherein the impactor supports a light source emitting alight beam having a known orientation relative to a longitudinal axis ofthe impactor.
 11. The computer-assisted surgery system according toclaim 10, wherein the given orientation has the light beam illuminatingthe medio-lateral axis of the pelvis, with a shaft of the impactor lyingin a plane of the light beam.
 12. The computer-assisted surgery systemaccording to claim 10, further comprising an ankle clamp having ankleinterfaces configured to remain fixed to the ankles, with linkagesinterconnecting the ankle interfaces, the ankle clamp further comprisingindicators for being illuminated by the light beam for recording themedio-lateral orientation.
 13. The computer-assisted surgery systemaccording to claim 12, further comprising a scale in the linkages tomeasure the distance.
 14. A computer-assisted surgery system foroutputting one of a leg length discrepancy and/or an offset between apre-implanting leg condition and a post-implant rejointing leg conditioncomprising: at least one instrument; at least one inertial sensor unitconnected to the at least one instrument, the inertial sensor unitproducing readings representative of its orientation; acomputer-assisted surgery processor unit operating a surgical assistanceprocedure and comprising a coordinate system module for setting a pelviccoordinate system from readings of the at least one inertial sensor unitwhen the at least one instrument is in a given orientation relative tothe pelvis, the pelvic coordinate system including a medio-lateral axisof the pelvis, a tracking module for tracking an orientation of the atleast one instrument relative to the pelvic coordinate system duringmovements thereof using the readings from the inertial sensor unit onthe instrument, and a geometrical relation data module for recordingbefore implanting an implant a medio-lateral orientation of the at leastone instrument representative of a medio-lateral axis of the legsrelative to the pelvic coordinate system and a distance between the legsalong the medio-lateral axis, for recording after implant rejointing themedio-lateral orientation and said distance, and for calculating a leglength discrepancy and/or an offset, based on said distances and saidmedio-lateral orientations; an interface for outputting the leg lengthdiscrepancy and/or the offset between the pre-implanting leg conditionand the post-implant rejointing leg condition.